We determined average cellular turnover rates by fitting mathematical models to 5-bromo-2′-deoxyuridine measurements in SIV-infected and uninfected rhesus macaques. The daily turnover rates of CD4+ T cells, CD4− T cells, CD20+ B cells, and CD16+ NK cells in normal uninfected rhesus macaques were 1, 1, 2, and 2%, respectively. Daily turnover rates of CD45RA− memory T cells were 1%, and those of CD45RA+ naive T cells were 0.5% for CD4+ T cells and∼ 1% for CD4− CD45RA+ T cells. In SIV-infected monkeys with high viral loads, the turnover rates of T cells were increased∼ 2-fold, and that of memory T cells∼ 3-fold. The turnover of CD4+ CD45RA+ naive T cells was increased 2-fold, whereas that of CD4− CD45RA+ naive T cells was marginally increased. B cells and NK cells also had increased turnover in SIV-infected macaques, averaging 3 and 2.5% per day, respectively. For all cell types studied here the daily turnover rate increased with the decrease of the CD4 count that accompanied SIV infection. As a consequence, the turnover rates of CD4+ T cells, CD4− T cells, B cells, and NK cells within each monkey are strongly correlated. This suggests that the cellular turnover of different lymphocyte populations is governed by a similar process which one could summarize as “generalized immune activation.” Because the viral load and the CD4 T cell count are negatively correlated we cannot determine which of the two plays the most important role in this generalized immune activation.